Windowed package having embedded frame

ABSTRACT

An integrated circuit (IC) package includes a mold compound, a die, and a window. The mold compound has a frame embedded within it. The frame has a top surface, a bottom surface, and a top-to-bottom opening therein. The die is attached to the mold compound, wherein the embedded frame lies below a periphery of the die. The window is attached to the mold compound and located above the die to allow light to reach the die.

This application is a continuation of Ser. No. 09/546,225, filed Apr.10, 2000, now U.S. Pat. No. 6,692,993, which is a divisional of Ser. No.09/219,186, filed on Dec. 21, 1998, now U.S. Pat. No. 6,072,232, whichis a continuation-in-part of application Ser. No. 09/172,734, filed onOct. 13, 1998, now U.S. Pat. No. 6,753,922, entitled “Image SensorMounted by Mass Reflow.”

BACKGROUND OF THE INVENTION

1. Field of the Invention

The described invention relates to the field of integrated circuitpackages. In particular, the invention relates to an integrated circuitpackage capable of being mounted to a circuit board via a mass reflowprocess.

2. Description of Related Art

A windowed integrated circuit package is used for various applicationsin which an integrated circuit is illuminated or irradiated by light orother radiation sources located outside the integrated circuit package.An image sensor is one use of a windowed integrated circuit package.

For example, a photodiode array may be placed within a windowedintegrated circuit package. The photo-detector array provides an imagedata output based upon the light incident on the photo-detector array.The photo-detector array may be used for capturing images or for otherimage reproduction applications. A color filter array (CFA) material isused with the photo-detector to filter the light impinging on the imagesensor to allow for the formation of full color images. Each filterallows a predetermined color of light to reach a correspondingphoto-detector, thus determining what color light will be sensed by thephoto-detector. By grouping sets of light sensors together, theintensity and color of light reaching an area can be determined.

Integrated circuit (IC) packages are mounted on circuit boards byvarious techniques including mass reflow and manual and hot barsoldering of the package to the circuit board. Manual soldering and hotbar soldering, however, are relatively slow and expensive processes.

Mass reflow board mounting is a faster, automated process. Mass reflowrefers to one of several different techniques that raise the temperatureof the IC package to approximately 215 to 225° C. At these elevatedtemperatures, solder residing on pads of the integrated circuit boardmelts and adheres to leads on the IC package. After the solder cools,the IC package remains firmly coupled to the solder pads. Mass reflowincludes infrared, convection, and vapor phase techniques.

Non-ceramic packages such as windowed plastic packages are moredesirable than ceramic packages because they are of lower cost thancorresponding ceramic windowed packages. However, until recently,standard windowed plastic packages tested on the mass reflow processexhibited problems such as cracked lids, delamination of the die fromthe die attach, and lid-sealant separation due to thermal expansionmismatch between the plastic and glass window. Until recently, thesewindowed plastic packages were mounted to circuit boards usingtechniques such as manual soldering that keep the package bulk fromreaching the elevated temperatures of the mass reflow process.

Co-pending U.S. patent application Ser. No. 09/172,710, entitled “MassReflowable Windowed Non-Ceramic Package,” assigned to Intel Corporationand Kyocera Corporation describes a windowed non-ceramic package thatdoes meet the thermal requirements for mass reflow board mounting.However, a way of reducing internal stresses is desirable. This wouldallow extended bake times to be reduced or eliminated, and largerpackage sizes to be mass reflowed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a cross sectional block diagram of the windowed quad flatpack (QFP) package 10 as modified by the Applicants.

FIG. 2A shows a schematic diagram of one embodiment of the package lid,including the ceramic frame and glass window.

FIG. 2B shows a side view of the package lid.

FIG. 2C shows another side view of the package lid.

FIG. 3A shows a schematic diagram of one embodiment of the entire ICpackage.

FIG. 3B shows a side view of the entire IC package.

FIG. 4 shows one embodiment of the process for attaching a die onto awindowed non-ceramic package.

FIG. 5 shows a cross sectional view of one embodiment of the moldcompound including an embedded frame.

FIGS. 6 and 7 show modeling of warpage patterns at 225° C. of an ICpackage with an embedded frame and without an embedded frame,respectively.

FIG. 8 shows one embodiment of an embedded frame that has cross barsthat join the sides of the embedded frame.

FIGS. 9A and 9B show one embodiment of a mold that can be used to embeda frame within the mold compound.

FIG. 10 shows an imaging system comprising an image sensor attached to acircuit board via a mass reflow process.

DETAILED DESCRIPTION

An improvement to a windowed non-ceramic integrated circuit (IC) packagecapable of being mounted via a mass reflow process is disclosed. A frameis embedded in the mold compound of the IC package. This reduces thestresses internal to the IC package when raised to the high temperaturesassociated with mass reflow.

Applicants have found that modifying a windowed plastic QFP packageavailable through Kyocera Corporation based in Kyoto. Japan allows theplastic package to withstand the mass reflow process without the lidseparating from the molded package or the die delaminating from themolded package. Additionally, the non-ceramic IC package may be combinedwith a CFA material with high temperature stability to produce an imagesensor that maintains its color performance despite being exposed to amass reflow process.

The following section describes the IC package capable of being massreflowed that is also described in co-pending U.S. patent applicationSer. No. 09/172,710, entitled “Mass Reflowable Windowed Non-CeramicPackage,” assigned to Intel Corporation and Kyocera Corporation. Thesubsequent section describes the improvement of adding an embedded frameinto the mold compound of the IC package.

Mass Reflowable Windowed Non-Ceramic Package

FIG. 1 shows a cross sectional block diagram of a windowed QFP package10 that is mass reflowable. A non-ceramic molded package 12 makes up thepackage body. For one embodiment, the non-ceramic molded package is madewith a low-moisture plastic, such as a low-moisture mold compound ofortho-cresol-novolac developed by Kyocera Corporation. For oneembodiment, depressions 22 indicate where ejector pins were used toremove the molded package after being formed. Appendix 1 includes anexample of the material characteristics of a low-moisture mold compoundof Kyocera Corporation.

Die attach 14 is used to hold the die 16 in place. For one embodiment,the die attach 14 is a low rigidity epoxy such as a silver-filled epoxymanufactured by Ablestik Electronic Materials and Adhesives, based inRancho Dominguez, Calif.

Wire bonds 18 attach the die 16 to a lead frame 20. The die attach 14 isselected to withstand the elevated temperatures of the mass reflowprocess. Delamination of the die 16 from the die attach 14 or moldedpackage 12 may be a problem during mass reflow. Applicants havedetermined that a two step cure process for the die attach, as will bediscussed with respect to FIG. 3, solves this problem.

A lid 30 seals the molded package. For one embodiment, the lid 30comprises a ceramic frame 32 made of alumina. The ceramic frame 32 holdsa transparent window. For one embodiment, the ceramic frame 32 includesa recessed ledge within which a glass window 34 resides. For oneembodiment, the molded package 12 and the ceramic frame 32 are sealedusing a bis-phenol A type epoxy. The epoxy seal may also be used to sealthe ceramic frame 32 to the glass window 34. Appendix 2 summarizes thecharacteristics of a bis-phenol A-type sealant that is suitable for usewith the present invention.

The modified windowed package is particularly suited for, but notlimited to, Complementary Metal Oxide Semiconductor (CMOS) image sensorsbecause their die size are relatively large (they can exceed 240 mils by240 mils). An embodiment of the package suitable for an image sensorincludes a window having a slightly larger area than that of thelight-sensitive portion of the die.

For one embodiment, the window is approximately 1.2 times the area ofthe light-sensitive portion of the die. The window size, however, variesdepending on its distance away from the die. FIGS. 2 and 3 showschematic diagrams of one embodiment of the lid and molded package.

FIGS. 2A–2C show a schematic of one embodiment of the package lid 30,including the ceramic frame 32 and glass window 34. The first dimensionis in mils, the dimension in parentheses is in millimeters. For oneembodiment, a glass window 34 is seated into a recessed ledge 40 in theceramic frame 32.

FIGS. 3A–3B show a schematic of one embodiment of the entire IC package50 in accordance with present invention. The first dimension indicatedfor an element is in inches, the second dimension (in parentheses) is inmillimeters. Although the embodiment shown includes a particular type oflead frame (quad flat pack—QFP), other types of lead frames may beemployed. Moreover, other packages may be used, including leadlesspackages such as ball grid array (BOA) packages, leadless chip carrier(LCC) packages, and leaded packages such as dual in-line package (DIP),and so forth.

FIG. 4 shows one embodiment of the process for attaching a die into awindowed non-ceramic package. At step 202, die attach is dispensed onthe molded package. For one embodiment, the die attach is comprised of alow rigidity epoxy such as a silver-filled epoxy, as previously stated.

The process continues at block 204, at which the die is scrubbed, ormoved back and forth while pressure is applied to firmly attach the dieto the die attach. Good adhesion of the die to a smooth surface of themolded package is achieved without plating the back-side of the die withgold.

The die attach is cured at block 206. It is important to eliminate voidsin the die attach, which may cause delamination problems. It has beenfound that a two stage cure process works better than a one stage cureprocess for eliminating voids in the die attach. For one embodiment, thedie attach is baked at approximately 100° C. for approximately one hour,then the die attach is baked at approximately 150° C. for approximatelyanother hour.

At block 208, wire bonds are attached between the die and the lead frameof the molded package.

At block 212, the lid is attached to the molded package. For oneembodiment, the lid comprises the glass window 34 attached to theceramic frame 32 with a his-phenol A type epoxy. The epoxy is cured bybaking. For one embodiment, the curing is performed by raising thetemperature to approximately 150° C. for approximately 70 minutes. Forone embodiment, the lid is attached to the molded package using the sameepoxy that was used to attach the glass window 34 to the ceramic frame32, and the epoxy is cured by also raising the temperature toapproximately 150° C. for approximately 70 minutes.

For one embodiment, the above steps are performed using laminar flowhoods in a clean room that meets level class sub 100, in which there isless than 100 particles of contamination of one micron or less per cubitmeter. This helps to prevent particle contamination of the die duringassembly.

For one embodiment, the window has a scratch-dig specification of 20microns. The scratch-dig specification denotes the largest allowabledefect in the glass. A larger defect may interfere with the imagingperformance of the image sensor.

In order to reduce moisture in the sealed package prior to performingthe mass reflow process, an extended bake cycle may be used just priorto bagging. For one embodiment, the sealed package is baked at 125° C.for 48 hours, then it is vacuum sealed in a moisture barrier bag forstorage or shipment. This allows the sealed package to meet theInstitute for Interconnecting and Packaging Electronic Circuits (IPC)level 4 surface mount requirements. (The IPC is a trade associationrepresenting over 2300 companies in the electronic interconnectionindustry worldwide.)

When the sealed package is ready to be mounted, it is removed from thebag and mounted to a circuit board using a mass reflow process, as shownat block 214. There are various types of mass reflow processes. For oneembodiment, an IR/convection mass reflow process is employed thatconforms to the following:

-   -   peak package body temperature of approximately 225° C.;    -   time above 215° C. is approximately 30 seconds;    -   time above 183° C. is approximately 140 seconds.

The windowed non-ceramic package is able to withstand the above massreflow process without the lid separating from the molded package or diedetaching from the molded package.

Mass Reflowable Windowed Non-Ceramic Package with Embedded Frame

FIG. 5 shows a cross sectional view of one embodiment of the moldcompound including an embedded frame 300. The mold compoundsubstantially surrounds the frame. (It may or may not be surrounded onthe bottom surface.) For one embodiment, the frame is made of ceramic tomatch the CTE of the ceramic frame bordering the window of the ICpackage. In another embodiment, the embedded frame is made of materialhaving a lower CTE than that of the mold compound. For example, theframe may comprise a copper/tungsten alloy or alloy-42 (an alloycomprising 42% nickel and 58% iron), which is frequently used in leads.

Because the stresses internal to an IC package are difficult to measure,the stresses have been modeled by finite element modeling, which iscommonly used as an engineering validation tool of mechanical designs.Table 1 shows exemplary values of the CTEs used in the modeling for themold compound, the die attach, the silicon die, the window sealant, thewindow, the ceramic frame, and the leads. Table 1 also shows values ofYoung's modulus (an indication of stiffness of the material) andPoisson's ratio (an indicative ratio of deformation in one directionwhen stretched in an orthogonal direction) which were used in themodeling.

TABLE 1 Materials properties at Different Temperatures Modulus, kN/mm²CTE, ppm/° C. 25 150 155 225 25 150 155 225 Poisson's Components ° C. °C. ° C. ° C. ° C. ° C. ° C. ° C. ratio*** Mold 14.4 14.4 2.1 2.1 11.411.4 48.1 48.1 .23 Compound* Die Attach* 8.0 .08 .08 .08 25 25 130 130.3 Silicon Die 130 130 130 130 2.6 3.2 3.2 3.6 .23 Window 4.2 4.2 .045.045 62.1 62.1 186 186 .35 Sealant** Glass 72.9 72.9 72.9 72.9 6.8 7.57.5 7.9 .208 Window Ceramic 280.5 275.6 275.6 272.6 5.74 6.76 6.76 7.38.25 Window Frame Lead (alloy 145 145 145 145 4.5 4.5 4.5 4.5 .3 42) Theglass transition temperature (Tg) for mold compound and sealant are 150°C. **Tg of Die Attach is 103° C. ***Poisson's ratio is consideredconstant from 25 to 225° C.

According to the above table, the materials properties of mold compound,die attach and sealant remains constant from 25 to its Tg, then changeto a different set of values from Tg+5° C. to 225° C. The Siliconproperties reflect those of [110] directions.

Table 2 is a summary of the modeled results of the reduction of internalstresses of an IC package using an embedded frame comprising alloy-42.The modeled results show that stresses in an IC package having theembedded frame are reduced significantly for both of the epoxyinterfaces between the backside die and the molded package and betweenthe lid and the molded package. This reduction in stress makes it easierto attach larger package sizes via mass reflow since larger packagesizes are more susceptible to internal stresses at temperature extremes,such as at mass reflow temperatures.

TABLE 2 Maximum Von Mises Max Von Mises Warpage in Package Stress in DieAttach, Stress in Window Bottom*, μm MPa Seal, MPa Temperature ° C. −65223 225 −65 25 225 −65 25 225 Without Embedded −45.4 −26.4 +60.0 179.3104.3 16.4 129.8 76.9 5.8 Frame With Embedded −23.4 −13.6 +13.3 136.579.4 4.8 133.2 77.4 4.7 Frame Difference, % 48.5 48.5 77.8 23.9 23.970.3 −2.6 −0.65 19.0 *+/−sign indicates the convex/concave warpage.

FIGS. 6 and 7 show modeled warpage patterns at 225° C. for an IC packagewith an embedded frame and without an embedded frame, respectively.Warpage is seen to be significantly reduced using an embedded frame.

The embedded frame may take on a variety of shapes. However, to reducecosts, a simple design such as a square loop or a miniature version ofthe window frame 32 of FIG. 2 may be used. Applicant has found the bestresults by the embedded frame overlapping with the periphery of the dieas shown in FIGS. 5 and 6.

For one embodiment, the embedded frame 300 may include cross bars 302joining the sides 304 for additional support, as shown in FIG. 8. In theminiature version of the window frame 32, a ledge holds the embeddedframe firmly in the mold compound. A more complicated shape of frame mayalso be used (for instance a honeycomb pattern), but the additional costof the frame should be low enough such that the overall package remainsless expensive than a flail ceramic package.

In another embodiment, a full glass window without a window frame isattached to the mold compound. An embedded frame reduces the stressesinvolved during the mass reflow process. For example, an embedded framemade of alloy-42 (CTE ˜4.5 ppm/C), a copper/tungsten alloy (CTE6.5˜ppm/C), or a ceramic frame (CTE ˜7.4 ppm/C) may be employed.

The Mold Process

FIGS. 9A and 9B show one embodiment of a mold that can be used to embeda frame within the mold compound. FIG. 9A shows a cross sectional sideview of the mold showing the placement of embedded frame 300 within themold cavity 340. FIG. 9B shows a top view of the mold cavitycorresponding to the dotted line 350 of FIG. 9A that looks down upon theembedded frame 300.

In this embodiment, stoppers 342 are used to hold the embedded frame 300in place. Runners and a gate 360 located on one side of the mold cavityprovides a source of the mold compound. The location of the runners andgate to the mold cavity 340 is designed such that the flow of moldcompound will apply pressure over the top surface of the embedded frameand hold the embedded frame down in the mold cavity 340. A vent 362 onthe opposite side of the mold allows air to be expelled as the moldcompound is supplied into the mold cavity 340. A lead frame 370 is alsoembedded when the mold compound is supplied to the mold cavity 340.

For one embodiment, multiple molded packages are made at the same time.The molded packages are joined together end-to-end in a strip, as iswell known. The molded packages are separated from another in anotherprocess step.

Imaging System

FIG. 10 shows an imaging system 400 comprising an image sensor utilizingthe IC package with an embedded frame. The image sensor is attached to acircuit board via a mass reflow process. The image sensor 410 isemployed as part of a camera, silicon eye, or other image device.Typically, the image sensor is electrically coupled to an imageprocessor 420 and a memory 430. The imaging system may also includeinterconnect circuitry 440 for communicating with other systems, such asa host computer system or other output device. The imaging system mayalso include a lens system (not shown) to focus the light on the imagesensor, as is well-known in the art.

The ability to attach the image sensor via the mass reflow processreduces costs and speeds up the manufacturing process. It also mayprovide a more reliable connection than manual soldering methods.

Thus, an IC package with an embedded frame capable of being mounted to acircuit board via a mass reflow process is disclosed. The specificarrangements and methods described herein are merely illustrative of theprinciples of this invention. Numerous modifications in form and detailmay be made without departing from the scope of the described invention.For example, with reductions due to the embedded frame of internalstresses at both the epoxy interfaces between the backside die andmolded package and between the lid and molded package, extended bakecycles may not be needed. Although this invention has been shown inrelation to a particular embodiment, it should not be considered solimited. Rather, the described invention is limited only by the scope ofthe appended claims.

1. An integrated circuit (IC) package comprising: a mold compound, themold compound having a frame embedded therein, said embedded framehaving a top surface, a bottom surface, and a top-to-bottom openingtherein; a lead frame; a die attached to the mold compound and to thelead frame wherein the embedded frame lies below a periphery of the die;and a window attached to the mold compound and located above the die toallow light to reach the die.
 2. The IC package of claim 1, wherein theembedded frame substantially comprises ceramic.
 3. The IC package ofclaim 1, wherein the embedded frame substantially comprises an alloy. 4.The IC package of claim 3, wherein the embedded frame substantiallycomprises alloy-42.
 5. The IC package of claim 1 further comprising: awindow frame bordering the window, the window frame having a CTE smallerthan that of the mold compound.
 6. The IC package of claim 5, whereinthe window frame is made of the same material as that of the embeddedframe.
 7. The IC package of claim 1, wherein the mold compound is aplastic.
 8. The IC package of claim 7, wherein the die is a color imagesensor die.
 9. The IC package of claim 1, wherein the die is attached tothe lead frame by wire bonds.
 10. An integrated circuit (IC) packagecomprising: a mold compound having a ceramic frame embedded therein; adie having a periphery, wherein the frame lies below the periphery ofthe die; and a window having a ceramic frame that is attached to themold compound and is located above the periphery of the die.